If the user of a PC wants to access the intranet, he needs a modem in his personal computer (PC). The modem allows the connection of the PC to a telephone or ISDN line, which constitutes the communications connection with a remote communications computer.
Definite connections should always exist or should be activated as needed. So that electrical current is not used unnecessarily in spite of these requirements, a sleep mode is provided for modern PCs, into which the PC is practically completely shut off, few device groups. However, the PC can be remotely turned on again by means of a wake-up signal.
This output characteristic is always present in the PC and means that the PC, which from the standpoint of the user is turned off, can actually always be remotely switched on, unless the user operates a mechanical network switch.
Modern PCs as a rule no longer have the classic arrangement of a switch connected with the electric power supply for disconnecting the device completely from the current supply. Instead, the power units of the PCs are continuously connected with the power supply and are activated and deactivated by means of semiconductor switches. The energy required for the activation and deactivation is also taken from the power supply. As a result, it becomes possible, as mentioned above, to switch the PC on or off for example, the electronic switch for the operational status of the power unit can be controlled by means of the modem. The modem itself receives its power via a different path, so that it is continuously switched on independently of the PC.
A PC can also be attacked via the telecommunications lines. Such attacks often occur when it can be assumed that the user cannot monitor his PC.